The Vertical Bridgman (VB) method is a known method for producing single crystals of calcium fluoride. The VB method involves moving a crucible carrying a calcium fluoride material vertically through a crystal growth furnace which has a temperature gradient near the melting point of the calcium fluoride material. In other words, first the crucible is raised to melt the calcium fluoride material, and then it is gradually lowered in temperature (i.e. cooled) for gradual crystallization from bottom to top to grow a single crystal of the calcium fluoride. In some methods, the calcium fluoride material is melted by simple temperature control of the heater of the crystal growth furnace, without raising the temperature of the crucible.
As an example of a crucible used for the VB method there may be mentioned the crucible described in patent document 1. The crucible disclosed in patent document 1 has a construction such that the calcium fluoride grows in a single crystal along the crystal plane of the seed (seed crystal), and it comprises a large-diameter starting material carrying section in which the calcium fluoride material is loaded, and a small-diameter seed carrying section situated below the starting material carrying section, wherein the calcium fluoride seed is loaded; the starting material carrying section and seed carrying section are connected via a tapered cone surface.